1. Field of the Invention
The present invention relates to an optical pickup apparatus for an optical recording and/or reproducing device. More particularly, the present invention relates to an optical pickup apparatus which is compatible with multiple types of media.
2. Description of the Related Art
An optical recording and/or reproducing apparatus records and/or reproduces information from an optical information storage medium (such as an optical disc) using a light beam focused by an objective lens. The recording density (or recording capacity) of the optical medium is determined by the size of the light spot produced by the objective lens. Equation 1 below shows the relation between the size of a light spot S, the wavelength (λ) of light, and the numerical aperture (NA) of the objective lens.S ∝/λNA   [Equation 1]
In general, to increase the area recording density of an optical disc, the size of the light spot formed on the optical disc needs to be reduced. As is evident by Equation 1, to reduce the light spot size, the wavelength should be short and the NA should be large. For instance, light with a short wavelength such as a blue violet laser and an objective lens with an NA of 0.6 or higher are required.
As is well known to those skilled in the art, a digital versatile disc (DVD) records and/or reproduces information using light with a wavelength of 650 nm (or 635 nm) and an objective lens with an NA of 0.6 (or 0.65 for a recordable-type DVD). Since a typical DVD is 120 mm in diameter and has a track pitch of 0.74 μm, its recording density on each side is greater than 4.7 GB.
Nevertheless, a DVD does not have sufficient capacity to record high definition movies. This is because at least 23 GB of recording density per side is needed to record a 135-minute-long high definition movie.
To meet the needs of high-density recording capacity, considerable research and studies are underway to develop and set standards for a high density optical disc, namely a next-generation DVD (hereinafter referred to as a HD-DVD), using light with a shorter wavelength than red light (that is, 405-408 nm), blue light for example, and an objective lens with an NA of 0.6 or higher and having a narrow track.
Meanwhile, to provide tolerance to accommodate for tilting of the optical disc, the thickness of the optical disc should be reduced if the NA of the objective lens is increased for high-density recording. Therefore, in consideration of an allowable tolerance due to tilting of the optical disc, the thickness of a conventional CD (1.2 mm) is reduced to 0.6 mm for a DVD and a HD-DVD. Also, the NA of an objective lens is increased from 0.45 for a conventional CD to 0.65 for a DVD and a HD-DVD. In the case of a HD-DVD, a blue light source will probably be used as a light source in view of its recording capacity. Thus, one problem in the development of an optical disc based on the new standards is the compatibility of the new optical disc with the existing optical disc.
A next-generation HD optical information recording storage medium called BD (Blue-ray Disc) using an objective lens with a 0.85 NA is actively under development. A BD uses a light source with a short wavelength (405-408 nm), that is, a blue light, and is 0.1 mm thick. This type of BD has about a 10 times greater recording density than that of a conventional DVD. Again, one problem in the development of an optical information storage medium with new standards is compatibility with existing optical information recording media.
For instance, among existing optical discs, write-once media such as DVD-Rs and CD-Rs use light sources with wavelengths of 650 nm and 780 nm because their reflectivity significantly decreases when different wavelengths are used. Therefore, if an optical pickup is designed to be compatible with DVD-Rs and/or CD-Rs, the optical pickup for a HD-DVD and the optical pickup for a BD must use two or three light sources with different wavelengths.
In practice, however, highly advanced technology is required to design and manufacture an objective lens which has a 0.85 NA and a working distance equivalent to that of the objective lens for a DVD and that at the same time is compatible with HD-DVDs, DVDs and/or CDs. Moreover, because there is no uniform standard yet for HD-DVDs and BDs using a short-wavelength (such as 405 nm), complicated optical systems and a large number of components will be required to manufacture an optical pickup which is compatible with high density recording media (both HD-DVD and BD) and conventional CDs and/or DVDs.
Accordingly, there is a need for an improved optical pickup apparatus which is compatible with different types of optical storage media.